A new scheme of differentiation within the thymus is described. Lyt 1 plus 23 minus cells comprise about 10% of normal adult thymus. Such cells are the earliest to appear in ontogeny. Lyt 1 plus 23 plus cells may be descendents, rather than progenitors of Lyt 1 plus 23 minus cells. A new alloantigen, Ly 9, with two alleles, Ly 9.1 and Ly 9.2, is described. Ly 9 is found on all lymphocytes. Certain autoantibodies are described. One, Au-1 detects a subset of bone marrow cells, all thymocytes, and is absorbable by brain tissue. Lymph node cells have been shown to interact synergistically with the 10% of thymocytes not agglutinated with peanut lectin in mixed lymphocyte reactions. AKTB-1, a lymphoma of AKR mice, has been separated by fluorescent activated cell sorting into a B cell line (IG plus) which grows in follicles of the spleen, and a T cell line (Ig minus) which grows in lymph node and in interfollicular areas of spleen.